Photocathode for electron tubes



Jun 1963 J. VAN LAAR ET AL PHOTOCATHODE FOR ELECTRON TUBES 2 Sheets-Sheet 1 Filed Nov. 18, 1965 FIG. 2

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INVENTORS JOHANNES VAN LAAR JACOB .LSCHEER BY M f AGENT June 4, 1968 J. VAN LAAR ET AL PHOTOCATHODE FOR ELECTRON TUBES 2 Sheets-Sheet 2 Filed Nov. 18, 1965 sdoo WAVELENGTH FIG.5

INVENTORJ JOHANNES VAN LAAR JACOB menses BY Cid-,6. ,F.

AGENT United States Patent 3,387,161 PHOTOCATHODE FGR ELECTRON TUBES Johannes van Laar and Jacob Jan Scheer, Emmasingel, Eindiioven, Netherlands, assignors to North American 1 Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Nov. 18, 1965, Ser. No. 508,499 Claims priority, application Netherlands, Dec. 2, 1964,

' 961 9 Claims. 01: 313-94 ABSTRACT OF THE DISCLQSURE This invention relates to a photocathode for an electron tube or device. In particular, it relates to a p-conductivity-type semiconductor photocathode or electron emitter activated with an alkali metal. An alkali metal is to be understood to mean herein also an alkali metaloxygen combination constituting a substance of low work function.

The prior art photocathodes of the aforementioned composition have a comparatively high red sensitivity and their maximum quantum efi'i-ciencies may amount on an average to 20%. For a photocathode of the composition Cs Sb, for instance, the red boundary lies well over 6300 A., and the maximum quantum efliciency of approximately 20% is attained after an abrupt rise of the sensitivity curve at approximately 4200 A. For a photocathode of the composition Na KSb-Cs, the red boundary extends further out to approximately 8500 A., but the sensitivity curve rises more slowly so that the maximum quantum efliciency of almost 40% is attained only at approximately 4200 A. That wavelength at which the quantum efiiciency amounts to 1% of the maximum efficiency of the sensitivity curve is assumed to be the red boundary. The red boundary consequently does not provide an absolute value for the sensitivity at that boundary. We have previously made some investigations on the photoelectric emission from p-type silicon, where we found that the red boundary of heavily-doped p-conducting silicon lies at approximately 5000 A., but the maximum quantum efficiency was much too low for practical use. This is reported in Philips Research Reports, 17, 101-124 (19-62).

The main object of our invention is to provide improved photocathodes which also have a favorable red boundary.

According to our invention, the photocathode comprises an alkali-metal-activated, heavily-doped, p-conductivity type IIIV compound or mixed crystals of such compounds with an energy gap lying between 1.1 electron-volts (e-v.) and 1.6 electron-volts (ev.). As usual, an I'IIV compound is understood to mean a practically equiatomic intermetallic compound of one of the elements (111) of the third column of the periodical system, viz., boron, aluminum, gallium, and indium, and one of the elements (V) of the fifth column of the periodical system, viz., nitrogen, phosphorus, arsenic, and antimony. In thecase of mixed crystals, the quantity of III atoms present is substantially equal to that of the V atoms. According to a further feature of our invention, this AIII-BV compound preferably consists of gallium arsenide having an acceptor concentration of at least 1X10 per cm. and preferably at least 1 l0 per cmfi. The maximum concentration amounts to approximately 1 'l0 A photocathode is thus obtained whose maximumquantum efficiency is at least equal to that of the known red-sensitive cathodes, and moreover the red boundary lies at a considerably greater wavelength than in the case of the known photocathodes. Still further, as a result of the abrupt rise of the sensitivity curve at the long-wave or red end, the overall sensitivity to white light is increased.

A preferred composition of our new photocathode is cesium-activated gallium arsenide containing 4X10 zinc atoms per cm. The red boundary then lies upwards of 9200 A. and the maximum quantum efliciency of ap proximately 40% is already attained at approximately 6000 A. It is also possible to incorporate in the semiconductor other acceptor elements which produce p-conductivity in gallium arsenide in addition to the zinc. Examples of other acceptor elements are cadmium, mercury, or tellurium. A preferred mixed crystal of the invention comprises gallium arsenide with at the most a few percent, viz., 10%, of gallium phosphide or gallium antimonide.

The invention will now be described more fully with reference to the drawing, in which FIGS. 1 to 4 show different electron tubes containing a photocathode in accordance with the invention, and FIG. 5 illustrates spectral sensitivity curves comparing the sensitivities of various photocathodes.

As is well known in the art, there are various kinds of electron tubes or devices that employ photocathodes. These include photocells, photomultipliers, image intensifiers, iconoscopes, orthicons, and others. In most cases, the photocathode is mounted within an envelope, which is usually evacuated, in a position to receive the energizing photons of the incident light or radiation. The electron e-missive part of the photocathode generally borders the evacuated space and emits electrons into it, which are collected by an anode or collecting electrode positioned a small distance away. The cathode and anode are connected across a source of potential and load in an external circuit. Where the photocathode is a crystal, the emissive surface may be a surface formed by cleaving the monocrystal along a crystal plane in a high vacuum or in air. Or the emissive surface may be formed by evaporating the material to form a thin layer on a conductive support. The alkali-metal activation, usually cesium, which has the lowest work function, is done in the conventional manner. A source of cesium is provided and cesium evaporated within the tube so as to form a deposit on the emissive surface of the cathode. Generally sufficient cesium is deposited until the photocathode exhibits a maximum sensitivity, which is usually achieved with an approximately monoatomic layer of the cesium.

FIG. 1 illustrates a tube of the type described in our previously-mentioned paper for making a crystal photocathode. It comprises a movable metal bellows 2 secured in a vacuum-tight manner to a quartz bulb 1. This metal part 2 supports a steel knife 3 by means of which part of a gallium arsenide monocrystal 4 is severed to expose a pure gallium arsenide surface. From a source (not shown), which may be of the type described in our previously-mentioned paper, an approximately monoatomic cesium layer 5 is applied to this freshly-cleaved surface by vaporization. A plate 6 serves as an anode. Upon exposure to radiation through the bulb 1, electrons are emitted from the cathode 4, 5.

FIG. 2 shows a modified phototube containing a curved molybdenum plate 8 arranged in an envelope. Onto the plate 8 is applied by vaporization a layer 9 of gallium arsenide containing /2 of gallium phosphide. This may -be done as follows. A fine granulated mixture of doped gallium arsenide and gallium ph-osphide is fed, in a good vacuum, into a heated tantalum crucible such that it immediately evaporates. This technique is known in the art as flush evaporation. The molybdenum substrate 8 is held at a temperature of about 500 C. to obtain the desired crystal structure. Afterwards, an approximately monoatomic cesium layer is applied to the gallium arsenide layer 9 to produce the finished cathode. The anode is denoted by 10.

FIG. 3 shows a photomultiplier tube comprising a glass envelope 11 on one end of which is deposited by vaporization a cesium-activated gallium arsenide layer 12 having a thickness of 1000 A. In the conventional way, the tube is also provided with a plurality of multiplier or dynode electrodes 13 and an anode 14 which constitute the remainder of the electrode system.

FIG. 4 illustrates an iconoscope with a gallium arsenide layer 16 deposited by vaporization on the flat end 15 of the envelope and serving as the light sensitive cathode. Of the remaining electrodes, only the thermionic cathode -17 is shown.

FIG. compares the spectral sensitivity of the photocathode of the invention with several prior art cathodes. In the graph, the quantum efficiencies are plotted along the ordinate as a function of the wavelength of the exciting light in A. The curve I was obtained with a monocrystal of cesium-activated gallum arsenide containing 4 10 atoms of zinc/cm. cleaved in a vacuum of torr. The curve II is typical of a Na KSb-Cs cathode, and the curve III is typical of a cathode Cs Sb. It is clearly apparent that the gallium arsenide photocathode of the invention has a particularly favorable quantum efficiency, a very long-wave red boundary and a steeply ascending sensitivity curve. The light outputs for white light for the photocathodes associated with the curves I, II and III are approximately 450 a/lumen, 150 ,ua/lumen and 50 nah/lumen, respectively.

While we have described our invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A photocathode of an electron tube having an emissive surface portion, said portion comprising a p-type semiconductive material having an acceptor concentration of at least 1x10 atoms/cm. and selected from the group consisting of a III-V compound and mixed crystals of III-V compounds and having an energy gap between 1.1 and 1.6 electron volts and activated with an alkali metal, where III is an element selected from the group consisting of boron, aluminum, gallium, and indium, and V is an element selected from the group consisting of nitrogen, phosphorus, arsenic, and antimony.

2. A photocathode for an electron tube having an emissive surface portion, said portion comprising cesiumactivated, p-type gallium arsenide having an acceptor concentration of at least 1X10 atoms/cm? 3. A photocathode as set forth in claim 2 wherein the acceptor concentration is at least 1X10 atoms/cm.

4. A photocathode as set forth in claim 3 wherein the acceptor is selected from the group consisting of zinc, cadmium, tellurium and mercury.

5. A photocathode as set forth in claim 3 wherein the gallium arsenide is doped with approximately 4X10 atoms of zinc/cmi.

6. A photocathode as set forth in claim 1 wherein the material is gallium arsenide with no more than 10 percent of gallium phosphide or gallium antimonide.

7. A photocathode as set forth in claim 1 wherein the photoemissive portion is a cleaved surface of a monocrystal.

8. A photocathode as set forth in claim 1 wherein the photoemissive portion is a vapor deposited layer.

9. An electron tube containing a photocathode as delined in claim 1.

References Cited UNITED STATES PATENTS 7/1963 Aigr-ain 313-346 12/1963 Stratton 313-646 JAMES D. KALLAM, Primary Examiner. 

